1. Field of the Invention
The invention relates to a dual clutch transmission for motor vehicles, having with two subtransmissions
2. Description of the Prior Art
Dual clutch transmissions are automatic shift transmissions which make it possible by means of two subtransmissions to carry out a fully automatic gear change without any interruption in force. The transmission control selects the gears automatically or according to the driver's wish within the framework of the permitted rotational speed ranges. The transfer of the torque takes place via one of the two clutches which connect the two subtransmissions to the engine. While one clutch closes, the other opens.
Particularly when such dual clutch transmissions are provided with a dry dual clutch, they often produce noise and vibration which arise mostly during crawling actions, when starting from a standstill, in load alternation processes and during gear changes. The main problems here are gear rattling, clutch grab during starting and the occurrence of shift shocks during gear changes. The causes of the problems listed are set out below:
A first problem, which occurs particularly in the case of dry dual clutches, is what is known as clutch fading which is partial or even complete clutch surface overheating which makes it impossible to control the clutch friction coefficient via the clutch slip. During the overheating phase of the clutch, clutch fading mostly results in a reduction in the friction coefficient and then, after clutch cooling, in an increase in the friction coefficient. Fluctuations in the friction coefficient also arise which may lead to shift shocks, shift slip and clutch grab during crawling or starting. The occurrence of clutch fading is mostly to be found in clutches which are not yet run in. The cause is usually geometric errors in the overall clutch system and its surroundings, with the result that insufficient contact surfaces between the friction disk, pressure plate and central disk may occur, so that the locally limited contact surfaces partially overheat.
A second problem is what is known as geometric grab caused by geometric clutch errors. This leads not only to torque irregularities or non-uniformities in the clutch parts and therefore to clutch grab during starting, but, as an effect, also to vehicle vibrations which are transferred from the clutch into the transmission housing and thus, via the engine and transmission suspension, into the passenger cell.
A third problem is unstable slip control which is likewise caused by geometric clutch errors. Short-circuiting of the engine side with the transmission side of the clutch may consequently occur, so that the torsional non-uniformities originating from the engine may lead to gear rattling.
A fourth problem is that where a clutch wobbles on the input shafts, with the result that the bearing clearances are enlarged, and therefore bearing damage may occur. By these geometric errors being generated, not only may noise and vibration nuisances arise, but there may even be a complete failure of the clutch via clutch contact and the overheating resulting from this.
A fifth problem arises while the clutch is being mounted on the driving disk. In this case, the clutch has to be lifted and pressed radially together with the driving disk. The clutch body must at the same time bear against the driving disk at right angles. This operation often leads to mounting errors and to a resulting oblique position of the clutch or to a situation where the clutch is not lifted or is askew. The consequence is a central plate which oscillates in the x/y/z direction which, in turn, triggers torque nonuniformities on the clutch. This may lead to the three causes of noise nuisance mentioned above.
A sixth problem arises due to the radial clutch torque modulation which occurs on account of torsional fluctuations in the friction coefficient and is transferred via a radial and axial component into the clutch output shafts and therefore into the transmission input shafts. If the pressure force of the corresponding clutch parts is lower than the axial force component, axial characteristic frequency excitations of the affected individual systems may occur, thus leading, in turn, to noise nuisances.
A seventh problem arises due to radial and axial excitation of the central plate when the overall pressure force of the clutches is minimal. This excitation may be an internal geometric clutch error or it may also be caused by the axial and radial torsional nonuniformities of the engine. This then leads, in turn, to axial clutch oscillation, which may give rise, in turn, to a transmission noise problem.
The abovementioned seven problems are usually the consequential effect of an oblique position and/or radial offset of the friction disks with respect to the pressure plates and the central disk. These are geometric errors which are caused by manufacturing inaccuracies and/or system errors, but may also be induced by errors in mounting the clutch itself.
With regard to the prior art, reference is made to DE 10 2005 037 514 A1 and to U.S. Pat. No. 6,830,140 B2 which relate to generic dual clutch transmissions in which the abovementioned problems arise.